Abstract

The pyramid sensor (PS) is an interesting alternative to the Shack-Hartmann wavefront sensor (SH WFS) for astronomical Adaptive Optics (AO) because of its potential advantages in sensitivity and applicability to novel wavefront sensing schemes. The PS uses a pyramidal prism to perform a knife-edge test in two dimensions simultaneously and relies on modulating the position of the prism to increase the linear dynamic range. It has been suggested that this could also be accomplished by a static diffusing element. We test this idea and show that the diffuser produces a modulation effect. We compare the results of our PS to a SH WFS measuring spatial and temporal properties of real turbulence produced in the lab with a hot-air turbulence generator.

J. M. Beckers, "Increasing the Size of the Isoplanatic Patch with Multiconjugate Adaptive Optics," in Proceedings of a ESO Conference on Very Large Telescopes and their Instrumentation, held in Garching, March 21-24, 1988,Garching: European Southern Observatory, pp. 693-703 (1988).

J. M. Beckers, "Increasing the Size of the Isoplanatic Patch with Multiconjugate Adaptive Optics," in Proceedings of a ESO Conference on Very Large Telescopes and their Instrumentation, held in Garching, March 21-24, 1988,Garching: European Southern Observatory, pp. 693-703 (1988).

Figures (7)

A schematic diagram of the PS implemented in 1d is shown. The prism spatially filters the electric field phasor of the aberrated beam in the focal plane while splitting the light into two beams. The images of the pupils are formed on the detector by a second lens. The signals are generated by the difference of the illumination in the top and bottom pupil, normalized to the total intensity.

A schematic diagram of the experimental setup is shown. Light from a fiber source is collimated and a tilt is introduced by the fold mirror. Using the beamsplitter the tilt was measured by the mini-Wavescope and the intensity in the re-imaged pupil of the PS was recorded. The turbulator was introduced into the setup by replacing the fold mirror and repositioning the source in line with the PS CCD.

This plot shows the fractional change in intensity in the re-imaged pupil of the PS recorded with the DALSA CCD. The x and y error bars are less than or equal to the size of the data points. The slope of calibration curve for the 0.5° diffuser is 1.62 ±0.03 millirad-1 and 0.89±0.01 millirad-1 for the 1.0° diffuser. The linearity of the tilt response demonstrates the modulation effect provided by the diffuser and the fact that the slope decreases when the modulation angle is increased illustrates that the sensitivity of the PS is inversely proportional to the modulation angle.

This plot shows the temporal power spectra of the tilt mode of the turbulence produced by the turbulator calculated using the mini-Wavescope and PS data at a temperature difference of ΔT ≈ 140°C. Fitting was performed to extract the knee frequency at the transition between the low frequency and high frequency regimes. This frequency is related to the effective wind speed, νeff, of the turbulator by equation 3.

In (a) the variances of the Zernike wavefront coefficient measured with the mini-Wavescope are shown. The results of Noll, 1976 were modified to included the attenuation of the inner and outer scale and the values for r0, l0, and L0 were extracted by fitting the modified variances. Panel (b) shows the Zernike slope variances measured with the PS. Again fitting is used to obtain r0, l0, and L0.

This plot shows the dependence of r0 on ΔT observed with the mini-Wavescope and the PS. The lines represent power law fits to the data with a fitting function of the form r0 = AΔT-b. The fit parameters are log(A) = 2.44 ±0.11 and b = 0.89 ±0.08 for the mini-Wavescope and log(A) = 2.36 ±0.13 and b = 0.91 ±0.09 for the PS. The exponent, b, agrees well between the two sensors meaning that both WFS’s measure the same trend in r0 vs ΔT

The distribution of sensitivity values for the PS using a diffuser is shown in this histogram. The mean value is 1.76 mrad-1, close to the overall calibration constant of 1.62 mrad-1 from Fig. 3. The standard deviation is 0.9 mrad-1.